1. Field of the Invention
The present invention relates to a voice coding apparatus which employs an analysis-by-synthesis coding technique that is one of voice coding techniques for efficiently coding a human speech.
2. Description of the Related Art
CELP (Code-Excited Linear Prediction) coding which uses linear prediction and an excitation code book is a typical analysis-by-synthesis coding technique. FIG. 13 illustrates the structure of a voice coding apparatus which uses this coding technique. In the diagram, an input speech x input to a speech input section 1 is supplied to a linear predictive analyzer 2 to acquire a linear prediction coefficient .alpha.. The coefficient .alpha., subjected to scalar quantization in a linear prediction coefficient quantizer 3, is supplied to a linear predictor 4. The linear predictor 4 receives an index i.sub.e of an excitation vector from the excitation code book 5 and outputs a linear predictive speech x.sub.v. A subtracter 8 obtains the difference between the input speech x and the linear predictive speech x.sub.v to acquire a predictive error e. This predictive error e is supplied via an aural weighting filter 6 to an error minimizer 7 to reduce the aural noise. The error minimizer 7 obtains the mean square error of the predictive error e, and holds the minimum mean square error and the index i.sub.e of the excitation vector at the time of this error. After the above processing is performed for every excitation vector in the excitation code book 5, the quantized linear prediction coefficient a and the index i.sub.e of the excitation vector are sent to a voice decoding apparatus.
The conventional voice coding apparatus could not minimize the linear predictive error sufficiently even when an adaptive code book that uses the correlation of the linear predictive errors between the adjoining frames is used.